HOW DO ORGANISMS REPRODUCE?

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HOW DO ORGANISMS REPRODUCE?

 

Introduction

Reproduction is a process by which living organisms produce new individuals of their own kind and maintain their existence generation after generation. Reproduction is not essential to maintain the life of an organism but it is essential to maintain life on earth and perpetuation of species from one generation to another.  Reproduction at its basic level (cellular reproduction) is involved in making similar or dissimilar body designs through the genetic material (DNA) present in the chromosomes of its nucleus.  DNA is the source of information for making proteins. Any change in the information leads to production of different proteins, which ultimately lead to altered body designs.  Basic event in reproduction is production of DNA copies in a reproducing cell. The process is called DNA replication. When the cell divides into two, each new cell gets a copy of each DNA or chromosome along with the whole cellular apparatus.

Complete accuracy in DNA copying leads to two exactly identical cells but any error in duplication can lead to dissimilar cells or variations.  The inbuilt tendency for variations during reproduction forms the basis for evolution.  Variations during reproduction enable the population of a species to get adapted easily to a particular inhabiting place/niche. Hence, reproduction is linked to the stability of populations of species.  Stronger variations are useful for the survival of species over time and enable the organisms to tide over any drastic alterations in their habitats.

 

Importance of Reproduction 

(i) Maintenance of the existence :- Organisms are maintaining the existence on the earth since their origin, million years ago only because of reproduction.

(ii) Preservation of species :- Species are preserved because of reproduction. It is possible because reproducing organisms produce new individuals which are very similar to themselves.

(iii) Role in evolution :- Some variations are produced in the new organisms during reproduction which play an important role in evolution.


Type of Reproduction
There are two  main methods of reproduction in living organisms.
(1) Asexual reproduction.
(2) Sexual reproduction.

 

(1) Asexual Reproduction:
Production of offsprings by a single parent without the formation and fusion of gametes is called

asexual reproduction.
It is a primitive type of reproduction in which offspring is produced by a cell or any vegetative organ of an organism .
In this type of reproduction offsprings are genetically identical to their parents.

Modes of asexual reproduction are fission, budding, spore formation, fragmentation, regeneration and vegetative propagation.

(i) Fission.
It is a kind of asexual reproduction in unicellular organisms to create two new individuals. It can be of two types:

(a) Binary fission.
One cell splits into two equal halves, e.g., many bacteria and protozoa like Amoeba, Paramecium and Leishmania.

Fig .1

(b) Multiple fission.
One cell divides into many daughter cells simultaneously, e.g., Plasmodium(malarial parasite), Amoeba in unfavourable conditions.
(ii) Budding : Process in which an outgrowth (bud) is formed on the body of parent organism which then detaches and become a new organism. e.g. Yeast and Hydra.

(iii) Spore formation: Spores are the microscopic asexual reproductive bodies with a thick wall.Spores are formed in ‘sporangium’.
Each spore on germination give rise to a new organism e.g. Rhizopus, Penicillium.

Fragmentation: In this process an organism breaks up into two or more fragments and each fragment develops into an adult organism. e.g. Spirogyra.

(v) Regeneration: The process of getting back a full organism from the body parts of the parent individual is called regeneration. Regeneration is carried out by specialised cells. e.g. Hydra, Planaria.

 

(vi) Vegetative propagation: This is an asexual method of reproduction in plants where vegetative parts namely root, stem and leaves give rise to new plants.

Vegetative propagation is of two types :
(A) Natural vegetative propagation
(B) Artificial vegetative propagation.

(A) Natural vegetative propagation: 
Plant reproduce without the help of human being.

By leaves: Leaves of some plants produce adventitious buds on their margin. Thus buds develop into new plants e.g. Bryophyllum, Kalanchoe.
By stem: In many plant, underground stems produce aerial shoots annually under favourable conditions e.g. Potato, Zinger, Onion, Grass.

      •  By roots: Roots produce adventitious buds which develops into new plants. e.g sweet potato.

        (B) Artificial vegetative propagation:  To prepare plants with desirable characters. These are of four types.

        (i) Cutting:
        In this
        method small part of plant is cut and buried partly in the moist soil then cutting develops roots and grows into a new plant. e.g. Rose, Sugarcane, Potato, Cactus.

        (ii) Grafting:
        Two plants of closely related varieties are joined together so that they live as one plant. The plant of which roots remain in the soil is called as stock. Cutting part of a plant that is grafted on the other rooted plant is called scion. e.g. Mango, Apple, Lemon.

        (iii) Layering:
        In this method a branch of the parent plant is buried in the soil. The portion of the branch which is contect with the soil produces roots and this rooted branch is called layer. Layer is then detached from the parent plant and act as a new plant. e.g. Jasmine, Hibiscus.

         (iv) Tissue culture or micropropagation: Cells or tissue which is isolated from the growing tip of plant called explant. The explant develops into undifferentiated mass of cells called callus in the proper culture medium. The callus is transferred to another medium containing hormones for growth and differentiation, that forms plantlet. The plantlets are transplanted into pot or soil to form mature plant. This technique is known as micropropagation. e.g. Orchids, Chrysanthemum.

Advantages of vegetative propagation 

It is a rapid, cheap and easy method of reproduction for the multiplication of plants.Disease free plants can be produced. Superior quality fruits or flowers can be produced by grafting. Genetically identical plants are produced.
Plants raised by vegetative propagation can bear flowers and fruits earlier than those produce from seeds.

Advantages of asexual reproduction

1. It isonly method of reproduction in most unicellular organisms.
2. The parental properties are
preseved.
3. It is
rapid method. Only one individual is enough.
4. It does not require any sexual maturation, production of gametes, transfer of gamete and their fusion.
5.
Number of methods available, according to convenience any method can be adopted. (2)

 

(2) Sexual reproduction:

It is a type of reproduction in which two different sexes (male and female) are involved. It involves the fusion of gametes from two different parents and results in the formation of new organism, which is genetically different from the parent.

 

Sexual reproduction in flowering plants

Sexual reproduction takes place through the agency of flowers in angiosperms (flowering plants). Flower is a specialized condensed reproductive shoot of flowering plants on which the essential reproductive parts are inserted.

A typical flower has four whorls arranged on the thalamus.

1. Calyx
It is the outermost whorl consisting of sepals. Sepals are green and leaf like structure.

Calyx protect the flower bud before it opens.

 

2. Corolla 
It is the second whorl, inner to calyx, consisting of petals.  Petals are generally large, coloured and showy. Corolla attract insects for pollination.

3. AndroEcium 
It is the third whorl, inner to corolla, consisting of male reproductive parts called stamens.  Each stamen has two parts – Filament and anther. Anther is lobed structure present at the tip of filament. Each anther has pollen sacs (microsporangia) which contain pollen grains (microspores). Each pollen grain produces two male gametes/ male germ cells.

4. Gynoecium 
It is the fourth and innermost whorl consisting of carpels. Carpel is present in the centre of flower. Each carpel has three parts –

Ovary, Style and Stigma.  Ovary is a swollen basal part of carpel. It contains ovules which are attached to placenta. Each ovule contain an embryosac that bears a haploid egg (female gamete). Style is the middle part of the carpel. It has stigma above it and ovary below it. Stigma is the apical part of carpel. It receives pollen grains.

Perianth:- If both sepals and petals are coloured and can not be distinguished from each other, then their whorl is known as perianth. Calyx and corolla are non essential parts of the flower because they are not directly involved in reproduction.

Bisexual flower:- When the male and female reproductive parts are present in the same flower are called bisexual flower e.g. Hibiscus, Mustard. Unisexual flower :- When the male and female reproductive parts are present in different flowers. e.g. : Papaya, Date palm, Mulberry, Gourd, Water melon.


Pollination 

Process in which pollen grains are transferred from the ripe anther to the stigma. It is of two types :

(i) Self pollination:– It is the transfer of pollen grains from an anther to the stigma of the same plant. If it is in the same flower it is called autogamy (e.g. Pea) and if it is between flowers of the same plant then it is called geitnogamy (e.g. Oxalis).

(ii) Cross pollination:- It is the transfer of pollen grains from anther to the stigma of different plants of the same species (e.g. Mango). Agencies of pollination :- Transfer of pollen from one flower to another is achieved by agents like wind, water, animals, insects and birds.

 

Significance of bright colour of flower:-
The bright colour of flowers is meant to attract insects which help in pollination. White colour shine in dark which attracts insects at night. Similarly, bright colour day-blooming flowers attract insects.

Fertilization 

Fertilization is the process of fusion of the male and female gametes, which takes place in the embryo sac present in the ovule. After pollination, pollen grains germinate on the stigma by producing pollen tube. The nucleus in the pollen tube divides into two male gametes. Pollen tube penetrates the stigma and passes through the style and enters the ovule through micropyle. It releases two male gametes in embryo sac.
One male gamete fuses with egg cell and second male gamete fuses with the two polar nuclei.
One male gamete + Egg cell    \buildrel {Syngamy} \over\longrightarrow    Zygote.
Second male gamete + Two polar nuclei   \buildrel {TripleFusion} \over\longrightarrow  Triploid nucleus (Primary Endosperm Nucleus) Syngamy + Triple fusion = Double fertilization.

Post fertilization changes in the flower 
Sepals, petals and stamen withers off. Style and stigma degenerates. Ovary develops into fruit. Ovule grows into seed.

See: SEeds are formed from ovule of a flower, it contains an embryo formed by repeated division of zygote, it is covered by seed coat which is formed from integuments. The endosperm are responsible for storage of food for growing embryo. Seeds develop into a seeding in appropriate conditions.
Endospermic seed: If endosperm is not consumed.
Non endospermic seed:– Endosperm may be consumed. Reproduction resulting from the fusion of male gamete and female gamete is called sexual reproduction.

OR

The type of reproduction in which fusion of male gamete & female gamete occur is called sexual reproduction.

 

Important features of sexual reproduction are given below :
(i) It involves two different parents i.e. one male and one female.
(ii) Each parent produces gametes.
(iii) Male gametes are called sperms while female gametes are called ova or eggs.
(iv) The fusion of male and female gametes is called fertilization. It results in to the formation of a single diploid cell zygote.
(v) The zygote undergoes repeated mitotic divisions to form embryo which differentiate to form full organism.
(vi) The organism produced in this type of reproduction are genetically different from both the parents and can resemble in certain features with parents.

Human reproductive system

Puberty:– The age at which the gametes and sex hormones to be produced and the boy and girl become sexually mature is called puberty.

Generally female pubertal age is 10-12 years, male pubertal age is 13-14 years.
Pubertal Changes (Secondary Sexual Characters) in Male :
Widening of shoulders.
Deepening of voice.
Growth of hairs under chest, armpits and around pubic area.
Appearance of beard and moustaches.
Growth of sex organs, [Testes & Penis].
Increased Activity of sweat and sebaceous glands.
Oily skin and appearance of pimples.
Darkening in skin colour of the genital area.
Pubertal Changes (Secondary Sexual Characters) in Female :
Widening of pelvis and hips.
High pitch voice
Growth of hairs under armpits and around pubic area.
Initiation of menstrual cycle.
Growth of mammary glands (breasts).
Darkening in skin colour of genital area.
Maturation of secondary sex organs like fallopian tubes, uterus.
Male Reproductive System

Fig.17

The male reproductive system consists of portions which produce the germ-cells and other portions that deliver the germ-cells to the site of fertilisation.  The formation  of germ-cells or sperms takes place in the testes. These are located outside the abdominal cavity in scrotum because sperm formation requires a lower temperature [1–3°C] than the normal body temperature. 
Testes secrete male sex hormone called testosterone. 
In addition to regulating the formation of sperms, testosterone brings about changes in appearance seen in boys at the time of puberty. These changes are called secondary sexual characters.
The sperms formed are delivered through the vas deferens which unites with a tube coming from the urinary bladder. The urethra thus forms a common passage for both the sperms and urine. Hence urethra is also known as urinogenital tract.
Along the path of the vas deferens, glands like the prostate gland and the seminal vesicle add their secretions so that the sperms are now in a fluid which makes their transport easier and this fluid also provides nutrition.  The sperms are tiny bodies that consist of mainly genetic material and a long tail that helps them to move towards the female germ-cell (ovum).

Female Reproductive System

The female germ-cells or eggs are made in the ovaries. They are also responsible for the production of female sex hormones i.e., Oestrogen and Progesterone.

When a girl child is born, the ovaries already contain thousands of immature eggs. On reaching puberty, some of these start maturing. One egg is produced every month by one of the ovaries.  The egg is carried from the ovary to the womb through a thin oviduct or fallopian tube. 
The two oviducts unite into an elastic bag-like structure known as the uterus. 
The uterus opens into the vagina through the cervix.
The sperms enter through the vaginal passage during sexual intercourse. They travel upwards and reach the oviduct where they may encounter the egg.
The fertilised egg (zygote) gets implanted in the lining of the uterus. The mother’s body is designed to undertake the development of the child. Hence the uterus prepares itself every month to receive and nurture the blood to nourish the growing embryo. The lining thickens and is richly supplied with blood to nourish the growing embryo. The embryo gets nutrition from the mother’s blood with the help of a special tissue called placenta. This is a disc which is embedded in the uterine wall. It contains villi. On the mother’s side are blood spaces, which surround the villi. This provides a large surface area for glucose and oxygen to pass from the mother to the embryo. The developing embryo will also generate waste substances which can be removed by transferring them into the mother’s blood through the placenta. The development of the child inside the mother’s  body takes approximately nine months. The child is born as a result of rhythmic contractions of the muscles in the uterus, called labour pain.

Since the ovary releases one egg every month, the uterus also prepares itself every month to receive a fertilised egg. Thus its lining becomes thick and spongy. This would be required for nourishing the embryo if fertilisation had taken place.  This lining is not needed any longer. So, the lining slowly breaks and comes out through the vagina as blood and mucous. This cycle takes place roughly every month and is known as menstruation. It usually lasts for about two to eight days.

 

Reproductive Health:

Sexually Transmitted Diseases (STDs)
There are many infectious diseases which are spread by sexual contact, called Sexually Transmitted Diseases (STDs) e.g. AIDS, Hepatitis. STDs occur mostly in the individuals who are involved in sexual activities with many partners.

Methods of prevention of STDs :
(i) The people should be educated about various STDs.
(ii) Extra marital relations should be avoided.
(iii) No sex without proper precaution.
(iv) High standard of moral education should be given to the people.

Methods adopted for population control:- The prevention of pregnancy in women is called contraception.

1. Planned control of population :
(i) By education people about the advantages of small family
(ii) Raising the age of marriage can help in reducing population growth.
(iii) By family planning.

2. Natural method:-
(i) Intercourse is safe for a week before and week after menstruation.
(ii) Coitus interruptus involves withdrawing penis before ejaculation.

3. Mechanical methods :
(i) It includes use of condoms which are the rubber or plastic sheets put on the penis before  coital activity. (ii) Use of diaphragms or cervical caps fitted in vagina of female to check the entry of sperms  into the uterus and also helps in avoiding conception.
(iii) Use of IUCD i.e., Intra Uterine Contraceptive Devices like copper-T and loops fitted in the uterus,  help to prevent fertilization. They can cause side effects due to irritation of uterus.

4. Chemical methods :
(i) It consists of using some chemicals which are spermicidal. They may be in form of tablets,  jellies, paste and creams introduced in the vagina before coital activity.
(ii) Another chemical method is the use of oral contraceptive (OC) pills which inhibit the secretion  of FSH and LH from the anterior lobe of pituitary gland and thus inhibiting ovulation from the ovary.
These contraceptive therefore change the hormonal balance so that egg cell are not released and hence prevent fertilization.

5. Surgical methods :
(i) Tubectomy involves cutting of fallopian tubes in females and Vasectomy involves cutting  of vas deferens of each side.
(ii) Removal of ovaries surgically is known as ovariectomy and removal of testes is known as  castration.
(iii) Another surgical method is MTP i.e. Medical Termination of Pregnancy or abortion.
(iv) Other method is tubal ligation in which fallopian tubes are blocked by an instrument called  laproscope.

After fertilization, membrane appears around the egg to prevent further entry of sperms. It is called monospermy. After the entry of sperm, ovum completes its maturation division.

Amniocentesis

Amniocentesis is a prenatal diagnostic technique to determine the genetic disorders, if any, of the foetus. Unfortunately, the useful technique of amniocentesis is being misused to kill the normal female foetuses as it can help to detect the sex of foetus also. Determination of sex by amniocentesis has been banned.

 


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